1. Field of the Invention
This invention relates to a refractometer, and more particularly to an apparatus for measuring refractive errors in human eyes.
2. Description of the Prior Art
Machines for manually measuring the refractive values (diopters) of the eyes have long been used and recently, various machines for automatically measuring such refractive values have been proposed.
An auto refractometer is constructed such that with a subject watching a visible object, infrared or near infrared beam is projected upon the retina and the reflected beam therefrom is detected by a photo-detector, and measurement is effected along a plurality of meridians (azimuths) to obtain information on the astigmatism of the eye.
U.S. Pat. Nos. 3,883,233 and 3,888,569 relate to an auto refractometer, and in the former, in order to obtain numerical values for a plurality of meridians, a test figure is rotated through 90.degree. about the optic axis and a focusing lens is reciprocated in the direction of the optic axis for each azimuth and the refractive value is measured from the position of the focusing lens when it is in focus. In the latter, a test pattern projection optical system is rotated through 5.degree. each about the optic axis and the focusing lens is reciprocated for each azimuth and the refractive value is measured from the position of such lens. In this latter apparatus, by paying attention to that the refractive error measured about the optic axis becomes coincident with a sine wave, six values are selected from among numerous measurement data and applied to the sine wave to thereby calculate out exact refractive values except for mechanical errors.
However, in accordance with the mathematical knowledge that the shape of a sine wave is determined by a minimum of three numerical values, the refractive error varied in the form of a sine wave may be determined by three measurement values.
U.S. Pat. No. 4,021,102 controls the position of a test mark by a servo mechanism such that the retina and the test mark are conjugate, and measures three values corresponding to three azimuths by successively selecting three sets of mirrors differing in direction of inclination provided in the projection optical system.
A first common nature of the above-described three patents is that the focusing lens or the test mark is reciprocated in the direction of the optic axis to render the test mark and the retina conjugate. Secondly, to vary the measurement azimuth, it is necessary to provide a mechanically movable member for rotating the test mark or the projection optical system or for sliding the mirror.
On the other hand, Japanese Patent Applications laid open under No. 73491/1975 and 138685/1975 reduce the measurement time by an apparatus which effects measurement by moving a photodetector only once in one direction during one measurement, to thereby reduce the pain of the patient or the subject who must otherwise watch a target for a long time and also reduce the error of measurement which may result from a variation in the refractive power of the eye being inspected during the measurement. However, due to the construction described in these laid open applications in which an image rotating prism provided, in the case of 138685/1975, in the projection optical system is rotated about the optic axis so that the refractive values of the eye about the optic axis are continuously measured, a great number of measurement values must be processed to calculate out information regarding the astigmatism, and also the provision of a mechanically movable member for varying the azimuth leads to the necessity of preventing the irregularity of the rotation of the means for driving such member.